The present invention relates to a high performance microwave filter. More specifically, the invention concerns the design and development of microwave filters particularly suitable for use in input or output multiplexers for broadband communications channels in satellite transmission systems, these filters being physically embodied by means of dielectric resonators included in metallic cavities of arbitrary shape, coupled with each other by means of windows, probes or loops.
The new demands for communications services in relation with multimedia applications make it necessary to employ increasingly wide communication channels in satellite transmission systems, which implies the use, in various subsystems of the satellite communications payload, of microwave filters with bandwidths several orders of magnitude greater than those habitually used up to now, that is, passing from the current relative bandwidths of approximately 0.6%, to bandwidths of 2% (bandwidths of 300 MHz to 14 GHz).
Said applications require tight electrical specifications that involve highly complex filter transfer functions, as well as requiring reduced dimensions and mass because they are intended for space applications.
There exist various conventional solutions that permit a larger bandwidth to be obtained, e.g. dielectric resonator filters and waveguide resonator filters. Nevertheless, said solutions have drawbacks, either for their poor electrical properties with regard to quality factor, temperature stability and close spurious signals (and consequently in-band distortion), or else by having relatively large dimensions and weights.
Filters based on dielectric resonators have been extensively employed in space applications for reasons of their low mass, high temperature stability of the electrical characteristics, and superior electrical properties with regard to their high quality factor, low spurious signals and facility for implementing complex transfer functions.
The monomode configuration that is habitually employed, is that based on the fundamental mode, mode TE01xcex4, and obtains the transmission and equalisation zeros through cross couplings, the couplings being implemented with irises, probes, loops, etc. The greatest difficulty with this technique lies in that in order to be able to attain the new bandwidths necessary (≈2% of relative bandwidth) recourse has to be made to geometries that consist in bringing the dielectric resonator positions closer together. These geometries have the drawback of having a poorer quality factor and greater variation with temperature of the electrical parameters in comparison with those employed for narrower bandwidths. In addition, due to the restrictions in design imposed by these geometries, it is impossible or very costly, from the point of view of design, factory production and adjustment, to guarantee the absence of spurious modes very close to or inside the passband, which in the end signifies that their electrical properties are degraded, impeding compliance with the specifications.
On the other hand, recourse has also been made to dual mode configurations, in which two modes are generated in a single cavity; some of said configurations being the following:
that which uses two degenerated HEM modes,
that which uses the TE01xcex4 and TM01xcex4 modes,
and that which uses the TE01xcex4 and the HEM11xcex4 modes, or to monomode hybrid filters also called xe2x80x9cmixed-modexe2x80x9d, which comprise some cavities working with the TE01xcex4 mode and other cavities working with the HEM11xcex4 mode, or whatever other combination of different modes. In this case, it is a question of a monomode configuration, understanding this to mean that in each of the cavities there only exists one mode.
These last two configurations (dual and mixed-mode) offer the same drawbacks as already expounded for the TE01xcex4 monomode configuration in relation with quality factor, poorer stability with temperature of the electrical parameters or distortion in the passband due to spurious modes very close to or inside the passband.
Another technique employed in the embodiment of microwave filters for space applications, with which filters having larger bandwidths are obtained, is that based on empty metallic cavities. This technique, however, suffers the drawbacks of filters with greater size and mass, if equivalent electrical properties are desired, and is more complicated to design than that with dielectric resonators.
Thus, it is necessary to facilitate a microwave filter of reduced size and weight, the configuration of which permits the design of filters with a very broad range of bandwidths, whilst having the excellent electrical properties required by the tight specifications for satellite communications channels. The high performance microwave filter of the present invention has the characteristics necessary to reach this objective.
The invention herein proposed permits the embodiment, in a simple manner, of microwave filters for communications channels in space applications reaching the bandwidths needed for the new requirements, especially those in relation with multimedia applications, which, with respect to the conventional channels known in this art, increase the bandwidth specifications by various orders of magnitude. These applications impose electrical specifications that imply the need to implement complex transfer functions which can include transmission and/or equalisation zeros.
The solution proposed by the present invention permits the bandwidth required by the new applications to be attained, whilst permitting a complex response and adequate properties, both in-band (variation of insertion loss, variation in group delay, etc.) and out-of-band (rejection), to comply with the tight electrical specifications of satellite communications channels. Said solution also retains the advantages of using filters based on dielectric resonators, that is, those that make possible filters of reduced size and mass, with high temperature stability and with a high value of quality factor.
The solution proposed by the present invention for achieving the features described consists of filters embodied by means of the coupled resonator technique. In the present invention, said resonators are of the monomode type, that is, in each resonator there is a single resonance at the central frequency of the filter (which is that which is used for obtaining the desired filter response) due to a single resonant mode that is the same for all the resonators, and the resonance products due to the remaining resonant modes are located at a frequency sufficiently removed as not to produce distortion in the desired filter response. Each one of said resonators (hereinafter composite resonator) is, in turn, formed by a metallic cavity and by a resonant element (also termed dielectric resonator) formed by a material of high dielectric constant situated in the center of the metallic cavity by means of a support formed by a material typically of very low dielectric constant. The dimensions and geometries of the metallic cavity, of the resonant element and of the support of the resonant element are designed in order to satisfy the following conditions:
in each composite resonator only one resonance is produced at the central frequency of the filter due to only one of the two originally degenerated orthogonal HEM11 modes, considering as such the modes which within the composite resonator have the electric field pattern shown in the figures (FIG. 1).
the resonance products due to the remaining resonant modes, including for example the HEM11 mode which is not employed for obtaining the filter response, are located at a frequency sufficiently removed as not to distort the desired filter response.
The couplings between the multiple composite resonators that can form the filter are embodied by means of capacitive irises, inductive irises, capacitive probes, inductive loops or other means of coupling, that is, which permit electromagnetic energy to pass from one composite resonator to another.
It also has an input coupling and another output coupling embodied by means of capacitive irises, inductive irises, capacitive probes, inductive loops or other means of coupling for permitting the entry of electromagnetic energy into a composite resonator and the egress thereof from a composite resonator other than that of entry.
Thus, an object of the present invention is that of providing a microwave filter comprising a plurality of composite resonators each one comprising a cavity and a dielectric resonator being housed within said cavity, and at least one coupling means between two composite resonators in adjacent arrangement, said composite resonators being of the monomode type and having a resonant frequency that corresponds to a mode of electromagnetic resonance of an hybrid electromagnetic family comprising electric field and magnetic field patterns, characterised in that:
at least two dielectric resonators of adjacent cavities are located in a same reference plane or in parallel reference planes, said reference plane being that which sections the dielectric resonator into two symmetrical halves and on which the field patterns of the two degenerated orthogonal modes are essentially equal turned through 90xc2x0 with respect to each other;
a respective originally degenerated resonant mode of each one of said resonators is perturbed by altering its resonant frequency; and
the filter offers a main path for the signal that traverses the composite resonators coupled in sequential manner and at least one alternative path for the signal provided by at least one cross coupling between two composite resonators spatially adjacent and not consecutive in the sequence that defines the main path of the signal.
According to one aspect of the invention, the respective electric field patterns of each substantially unperturbed resonant mode of said composite resonators are in a parallel arrangement.
According to another aspect of the invention, said respective field patterns of the substantially unperturbed modes are oriented in such a manner that the directions of the electric field in the center of the composite resonators are also arranged perpendicular to the direction of a coupling furnished by a coupling means between said resonators.
According to another aspect of the invention, said respective field patterns of the substantially unperturbed modes are oriented in such a manner that the directions of the electric field in the center of the composite resonators are parallel and perpendicular to the plane that traverses a probe that serves as a coupling means between said resonators.
According to another aspect of the invention, said perturbation provoking a separation in resonant frequency of the orthogonal modes is obtained in the composite resonators of the filter by means of a cavity of asymmetrical geometric shape or of symmetrical geometric shape with an aspect ratio between the dimensions on the different axes of symmetry other than unity.
According to another aspect of the invention, said separation of orthogonal modes in the composite resonators of the filter is obtained by means of asymmetrical or off-centered arrangement of the dielectric resonator in a cavity.
According to another aspect of the invention, said separation of orthogonal modes in the composite resonators of the filter is obtained by means of positioning an adjustment element, like a slug or a post, arranged in an off-centered manner with respect to the center of the composite resonator.
According to another aspect of the invention, said separation of orthogonal modes in the composite resonators of the filter is obtained by means of whatever combination of the aforementioned perturbations.
According to another aspect of the invention, said separation of orthogonal modes is obtained by using composite resonators of different types from among those described above.
This and other characteristics of the invention are described in greater detail below with the help of the drawings attached.